Abstract Body

BiS₂-based compound LaOBiS₂ shows superconductivity at about 3 K via the partial elemental substitution of Fluorine (F) for Oxygen (O), corresponding to an electron carrier doping to a conduction BiS₂ layer [1]. The superconducting transition temperature (T_c) increases by partial elemental substitution, indicating this material is sensitive to structural distortion. Recently, it was found that the partial substitution of lead (Pb) for bismuth (Bi) increases T_c to about 5 K [2]. The Pb concentration is below 15 %. Furthermore, this substitution also induces anomalous behavior in the temperature dependence of electrical resistivity. This electrical resistivity anomaly has been reported to be a structural phase transition to a low-symmetry structure [3]. Although the transition relates to the enhancement of superconductivity, the change of detail properties has not been understood. Therefore, it is important to clarify the Pb-substitution effect in order to understand the superconducting mechanism of BiS₂-based compounds.

In this study, we synthesized polycrystals of LaO_0.4F_0.6Bi_1-xPb_xS₂ and investigated their thermoelectric properties using the thermal transport option (TTO) for PPMS to clarify the Pb-substitution effect.

Polycrystalline samples of LaO_0.4F_0.6Bi_1-xPb_xS₂ were prepared by the solid-state reaction.

Pb-substituted samples were successfully synthesized because XRD patterns for all samples could be indexed with the structure of LaOBiS₂. Electrical resistivity measurement revealed that superconductivity appears for x=0.12, not seen for x=0.08, 0.18 (Fig 1(a)). In addition, the anomaly was observed above x=0.12. This result is consistent with previous study [3]. On the other hand, the absolute value of the Seebeck coefficient at room temperature increases, indicating that hole carriers are monotonically doped via Pb-substitution (Fig 1 (b)). Moreover, it was found that the value of thermal conductivity is more significant for x=0.12 and smaller for x=0.18 compared to x=0.08 (Fig 1(c)), suggesting that the disorder of the crystal structure is enhanced at x=0.18 in comparison with x=0.12. These results indicate that the T_c of the Pb-substituted La(O,F)BiS₂ depends not only on the electron carrier concentration but also on the disorder of the crystal structure. In the presentation, we will report the evaluation of physical properties in LaO_0.4F_0.6Bi_1-xPb_xS₂ in detail.

References

[1] Y. Mizuguchi et al, J. Phys. Soc. Jpn. 81, 114725 (2012).
[2] S. Otsuki et al, Solid. State. Commun. 270, 17-21 (2018).
[3] S. Okada et al, J. Phys. Soc. Jpn. 93, 064701 (2024).

Acknowledgment

We performed their thermos electric properties using the thermal transport option (TTO) for PPMS at Nano Frontier Superconducting Materials Group in NIMS. We appreciate the experimental support by Mr. Okabe and Ms. Yamashida.

Figure 1 Temperature dependence of the (a) electrical resistivity, (b) Seebeck coefficient and (c) thermal conductivity

Keywords: BiS₂-based superconductor, Elemental substitution, Structural phase transition, Thermoelectric properties